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Human Vaccines & Immunotherapeutics Volume 18, 2022 - Issue 6
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Acceptance – Research Article

Vaccine confidence among people who use drugs: A cross-sectional survey

Paula M. Frewa Schools of Public Health, Medicine, and Integrated Health Sciences, University of Nevada, Las Vegas, NV, USA;b Population Health & Health Equity Initiative, University of Nevada, Las Vegas, NV, USAView further author information
,
Jay T. Schamela Schools of Public Health, Medicine, and Integrated Health Sciences, University of Nevada, Las Vegas, NV, USA;b Population Health & Health Equity Initiative, University of Nevada, Las Vegas, NV, USAView further author information
,
Laura A. Randalla Schools of Public Health, Medicine, and Integrated Health Sciences, University of Nevada, Las Vegas, NV, USA;b Population Health & Health Equity Initiative, University of Nevada, Las Vegas, NV, USAView further author information
,
Adrian R. Kinga Schools of Public Health, Medicine, and Integrated Health Sciences, University of Nevada, Las Vegas, NV, USA;b Population Health & Health Equity Initiative, University of Nevada, Las Vegas, NV, USAView further author information
,
Anne C. Spauldingc Department of Epidemiology, Rollins School of Public Health, Emory University, Atlanta, GA, USA;d Department of Medicine, Division of Infectious Diseases, Emory University School of Medicine, Atlanta, GA, USAView further author information
,
Elizabeth Wue Department of Social Welfare, UCLA Luskin School of Public Affairs, Los Angeles, CA, USA;f Southern California HIV/AIDS Policy Research Center, UCLA Luskin School of Public Affairs, Los Angeles, CA, USA;g UCLA Hub for Health Intervention, Policy, and Practice, UCLA Luskin School of Public Affairs, Los Angeles, CA, USAView further author information
&
Ian W. Hollowaye Department of Social Welfare, UCLA Luskin School of Public Affairs, Los Angeles, CA, USA;f Southern California HIV/AIDS Policy Research Center, UCLA Luskin School of Public Affairs, Los Angeles, CA, USA;g UCLA Hub for Health Intervention, Policy, and Practice, UCLA Luskin School of Public Affairs, Los Angeles, CA, USACorrespondence[email protected]
ORCID Iconhttps://orcid.org/0000-0001-7454-8632View further author information
ORCID Icon show all
Article: 2123201 | Received 31 May 2022, Accepted 07 Sep 2022, Published online: 28 Sep 2022

ABSTRACT

Adult immunization coverage remains low in the US, particularly for people who use drugs (PWUD), a population that experiences a disproportionate burden of vaccine-preventable diseases. The extent of and characteristics associated with vaccine confidence (VC) held by PWUD is poorly understood. As VC strongly correlates with vaccine uptake, this cross-sectional study identifies mutable factors associated with VC and quantifies its relationship to immunization status within a highly vulnerable, underimmunized population of PWUD. Using a community-engaged research strategy with select partner organizations hosting syringe exchange programs in Atlanta, Los Angeles, and Las Vegas, USA, we surveyed participants ages 18–69 years served by these organizations from 2019 to 2020. Survey measures included sociodemographics, health behavior including immunization receipt, and vaccine confidence in adult vaccinations using a modified Emory Vaccine Confidence Index (EVCI). The findings reflect relatively low VC among the 1,127 recruited participants, with 56% expressing low VC (EVCI 0–12), 35% medium (EVCI 13–20) and 10% high (EVCI 21–24). EVCI varied by city, with lowest confidence in Atlanta and highest in Las Vegas. VC was associated with past receipt of specific vaccines, including hepatitis A, MMR, Tdap, and influenza. VC varied by specific sociodemographic correlates such as housing insecurity (reduced confidence) and receipt of public benefits or disability (increased confidence). This study identified correlates associated with VC based on site and sociodemographic characteristics for this priority population, highlighting the need for specific interventions to raise VC among PWUD, especially among those experiencing housing insecurity and without public benefits.

Introduction

The achievement of improved US adult immunization coverage rates features in Healthy People 2030 goals,Citation1–3 with considerable attention given to addressing social and structural determinants contributing to suboptimal coverage rates for recommended vaccinations (i.e., hepatitis A (HAV) and hepatitis B (HBV), pneumococcal disease, and influenza (flu) vaccines).Citation4–6 Among US adults, people who use drugs (PWUD) shoulder a particularly high burden of vaccine-preventable diseases (VPD) and are especially at risk for diseases such as meningitis, HAV, HBV, and invasive pneumococcal disease.Citation4,Citation7 Hepatitis A incidence in particular has increased more than 5-fold since 2016, with much of the increase affecting susceptible PWUD.Citation8,Citation9 Additionally, PWUD experience greater lifetime health disparities associated with HIV and HCV infection compared to healthy adults.Citation10–13 Recent immunization data also reflect lags related to SARS CoV-2 vaccination and booster rates within communities with health disparities.Citation14–18 The social,Citation19,Citation20 healthcare,Citation21,Citation22 and economic costsCitation23,Citation24 associated with suboptimal adult immunization coverage in this highly vulnerable population cannot be overstated.

Few studies have explicitly explored why PWUD rank among those with lower vaccination rates.Citation25 Contextual factors linked to immunization status among PWUD include social marginalization, low healthcare access and utilization, risk denial, and transitory or insecure life circumstances.Citation26 However, the COVID-19 pandemic ushered in a new vaccination era wherein the role of VC in decision-making had never been more prominent in the public consciousness.Citation17,Citation27,Citation28 Scant evidence has been gathered to understand how VC is shaped under normal conditions (i.e., pre-COVID) and the extent to which it contributes to vaccine decision-making of adults facing health risks.Citation29 As COVID-19 vaccinations rolled out, understanding VC gained importance to improve initial uptake and shape address novel methods of curtailing eroding adult vaccination rates.Citation30,Citation31

Vaccine confidence has been defined in many ways.Citation32–37 It has been characterized as a volatile construct that operates differently across populations and contexts.Citation38 Fundamentally, VC reflects a set of attitudes, values, and knowledge about vaccines, broader immunization issues, and trust in the people and systems guiding vaccine development and recommendations.Citation39,Citation40 Given the need to develop measures to assess VC for detection of changes in immunization trends, our team previously developed Vaccine Confidence Index (VCI) measures with two discrete populations – parents of children >7 years and adult men-who-have-sex-with-men.Citation41,Citation42 These initial measurement efforts found that higher VC corresponded with improved vaccine uptake across these population segments.Citation41,Citation42 We also identified specific factors (e.g., peer influence) that influenced VC in vaccines and in the vaccination system more broadly.Citation43 The lessons learned have contributed to the development of novel interventions to address VC determinants associated with lower immunization coverage that are specific to the needs of the target population.Citation44–46

Given these findings, and the current COVID-19 environment wherein the achievement of society-wide vaccine equity is an important goal,Citation47–49 we set out to gain additional insight into VC of PWUD, an overlooked population facing significant health and social challenges: substance abuse,Citation50 high-risk sexual behavior,Citation51,Citation52 homelessness,Citation53,Citation54 mental illness,Citation55 violence,Citation56 and incarceration.Citation57 PWUD stand to benefit from improved adult vaccine uptake. To our knowledge, no prior research has characterized or estimated VC among PWUD. We consider this exploration to be of moral and ethical importance if we seek to achieve greater vaccine equity.Citation58 Strategies focused on improving VC will not only inform public health practice and policy, but also mitigate the risks faced by those who delay and/or refuse vaccines by informing practical, feasible approaches to improve VC among those often left behind.Citation59,Citation60 This exploratory research therefore examined critical social and structural determinants of health that figure prominently in the literature examining PWUD health outcomes,Citation61 guided by a socioecological conceptual framework to investigate the influences that shape vaccine confidence and immunization outcomes.Citation62,Citation63 Accordingly, the questions that guided our inquiry included the following:

  1. What is the estimated level of vaccine confidence among adult populations of PWUD?

  2. How does vaccine confidence correlate with recommended vaccine receipt in this population?

  3. What correlates are associated with vaccine confidence in this population?

Materials and methods

Study design and sample

Vaccine confidence among PWUD was surveyed as part of a larger cross-sectional study of health behaviors of PWUD in the United States.Citation46,Citation64 Atlanta, Georgia, Los Angeles, California, and Las Vegas, Nevada were chosen as project locations based on characteristics of the populations of PWUD living in those cities, including vulnerability to HIV, viral hepatitis, and other vaccine-preventable diseases. Project protocols were approved by the University of Nevada Las Vegas Institutional Review Board (IRB), as well as independent determinations for secondary analyses by the IRBs of Emory University, the University of Nevada Las Vegas (#1428464-5), and the University of California Los Angeles (#19-001173).

Selection of the partner organizations and recruitment and enrollment of participating clients were implemented using purposive sampling. Partners were selected from well-established organizations with at least two years of syringe service operation, diverse clientele, and a variety of service modes, including brick-and-mortar, mobile outreach, and mail provision. Participants were recruited in-person by trained project staff from May 2019 through February 2020. Eligible participants had accessed the organization’s syringe services at least once and reported use of opioid drugs in the preceding six months, were residents of counties serviced by partner organizations, were ages 18–69, and were able to comprehend and answer survey questions in English or Spanish. Potential participants were excluded if they did not meet the eligibility above eligibility criteria or could not or chose not to provide voluntary informed consent.

Following the in-person screening and consent process, participants completed a self-administered Qualtrics-based electronic survey using a supplied Wi-Fi-enabled iPad and received nominal health promotion and well-being items (e.g., lotion, sunscreen, socks, umbrellas) as compensation. A total of 1,127 partner clients completed surveys, out of 1,368 recruited for screening. As described in the survey protocol, the sample size was chosen for adequate margin of error in estimation and subpopulation analyses and to fit logistical constraints.Citation64 Harrell’s conservative rules for limiting sample size suggest that this sample could support over 50 predictors in a linear regression, or 17 in a logistic regression with an outcome rate of 30%.Citation65

Measures

Vaccine confidence (VC)

Vaccine confidence was measured using the eight-item Emory Vaccine Confidence Index (EVCI), previously developed by members of our team and validated for childhood vaccinations.Citation41 Item wording was modified to apply to immunizations recommended for adults. Individual EVCI items, possible responses, and corresponding EVCI scores are displayed in Table S1 in the Appendix. Four items asked participants to rate how strongly they agreed with given statements and had five response options ranging from “Strongly Disagree,” to “Strongly Agree.” Four additional items asked participants to rate their level of trust in given people or organizations and had five response options ranging from “1 – No Trust” to “5 – Complete Trust” along with two additional options for “Don’t Know” and “Don’t Use.” To compute an index score for each participant, individual items were scored from 0 to 3 points. For strength of agreement items, “Strongly Disagree” and “Disagree” responses were scored as 0, “Neutral/Not Sure/Do Not Know” as 1, “Agree” as 2, and “Strongly Agree” as 3. For items that asked participants to rate trust, “1 – No Trust,” and “2” responses scored 0, “3,” “Don’t Know” or “Don’t Use” scored 1, “4” scored 2, and “5 – Complete Trust” scored 3. The overall EVCI index score was computed on a scale from 0 to 24 as the sum of these individual item scores. EVCI scores are also interpreted categorically as “high confidence” (21–24), “medium confidence” (13–20), and “low confidence” (0–12).

To assess the internal structure of the modified EVCI within our sample, a confirmatory factor analysis (CFA) was performed using the lavaan package in R version 4.2.0 (R Foundation for Statistical Computing, 2022). The model with all items loading on a single latent variable indicated the presence of additional structure (RMSEA = 0.261 [95% CI: 0.250 to 0.273], SRMR = 0.134, CFI = 0.69, TLI = 0.57). A follow-up exploratory factor analysis was performed, with the scree plot indicating a two latent-factor model with one additional item in a third domain. The three items loading on the first factor were related to personal vaccine experiences and trust (“Vaccines recommended for adults are safe,” “My doctor or nurse is a reliable source for trustworthy vaccine information,” and “My doctor or nurse has my best interest in mind when making vaccine recommendations”), so this factor was labeled “Personal Trust.” The four items loading on the second factor were related to trust in institutions making and promoting vaccines, so was labeled “Institutional Trust.” The additional item was “It is important for everyone to get the recommended vaccines for adults.” A CFA performed on this model indicated adequate fit (RMSEA = 0.075 [95% CI: 0.063 to 0.087], SRMR = 0.035, CFI = 0.98, TLI = 0.97). Standardized factor loadings and correlations are shown in Table S2 in the Appendix. A likelihood ratio test favored the three-domain model (Chi-square difference = 1410.5, df difference = 2, p < .001). Both the unitary and three-domain models demonstrated high internal consistency (unitary Cronbach’s alpha = 0.874; “Personal Trust” alpha = 0.873, “Institutional Trust” alpha = 0.894).

The three identified domains reflect domains identified in the National Vaccine Advisory Committee’s (NVAC) definition of vaccine confidence (trust in recommended immunizations/providers, and trust in the process of vaccine development and recommendation), together with an additional domain of vaccine importance.Citation66 These domains were incorporated in the development process of the EVCI.Citation41 While the factor analyses indicated additional structure in the EVCI, the overall EVCI construct index score was used in analyses for this study due to the exploratory nature of the analysis, the design of the index as a multi-domain construct to assess a vaccine confidence construct in line with NVAC definition, adequate correlation between factors (0.45), and high internal consistency.

Vaccine receipt

Participants were asked to self-report vaccination receipt for the HAV, HBV, Flu, Measles, Mumps, Rubella (MMR), and Tetanus, Diphtheria, Pertussis (Td/Tdap) vaccines; all vaccines that are recommended for adults who use or inject drugs. Participants could indicate that they had received, had never received, or were unsure of receipt for each vaccine. Since vaccination counseling would be recommended for PWUD who are unsure of their vaccination status, for this study “unsure” responses are considered non-receipt.

Sociodemographic and drug use items

Several sociodemographic survey items were incorporated into this study, including age, race/ethnicity, gender identity, sexual orientation, education, household income, primary source of income, homelessness or housing insecurity, insurance status, incarceration history, and IV injection drug use. A dichotomous indicator for any IV injection drug use in the past 6 months was aggregated from items asking participants to report drug use methods for 13 drug categories: methadone, opiates/analgesics, barbiturates, sedatives, crack cocaine, powder cocaine, prescription amphetamines, street amphetamines, cannabis, hallucinogens, inhalants, spice, and bath salts.

Statistical analyses

Sociodemographic characteristics of the sample were reported overall and for each study site, and differences between sites assessed using chi-square tests with list-wise deletion of missing items. The mean and standard deviation of EVCI scores were computed for participants who completed all eight EVCI items, and mean scores and proportions of high, medium, and low EVCI scores estimated. Estimating confidence class proportions allowed qualitative interpretation of the level of vaccine confidence in our population.

To incorporate information in partially completed EVCI scales and to better incorporate completed items for other variables in multivariable analyses, a multiply imputed dataset was constructed for use in subsequent analyses. A fully conditional specification (FCS) imputation model was used to create 100 imputed datasets, iterating 100 times between datasets. All sociodemographic and risk factors used in analyses were included into the imputation model along with the eight EVCI items. Derived metrics were computed following imputation within all imputed datasets, including overall EVCI index scores, EVCI score classes (high, medium, low), and IV injection drug use. The pseudo-random number generator was seeded from the true random number generator provided by RANDOM.ORG.Citation67

Using pooled estimates computed from these multiply-imputed datasets, the association between VC and vaccine receipt was explored by estimating vaccine receipt for participants in each vaccine confidence class (high, medium, low) for each of the five assessed vaccines. Pairwise vaccine receipt rate comparison tests were performed between confidence classes for each vaccine. Additionally, the odds ratio for increase in odds of vaccine receipt for each point increase in EVCI score was estimated for each vaccine using logistic regression, allowing for finer assessment of potential associations. Unadjusted odds ratios were reported together with odds ratios adjusted for sociodemographic factors. Factors associated with VC were explored through bivariate linear regressions, with mean EVCI scores reported across levels of sociodemographic and other risk factors. Finally, a full multivariable linear regression was fit to assess conditional differences in EVCI scores after adjustment for other factors.

For these analyses, sufficiency of evidence was considered at type 1 error rate threshold of ⍺ = 0.05 and confidence intervals computed at a 95% confidence level. SPSS 25.0 software (IBM Corp, 2017) was used for all data cleaning and analysis tasks.

Results

Sociodemographic characteristics

displays sociodemographic and risk-factor characteristics of the 1,127 survey participants, overall and stratified by study location. Study participants were drawn across all age groups, with 22% (252/1127) aged 18 to 30, 52% (590/1127) aged 31 to 50, and 25% (285/1127) aged 51 or older. A greater proportion of Los Angeles participants were 51 or older (44%; 204/414) compared to 8% (31/465) in Las Vegas and 20% (50/248) in Atlanta.

Table 1. Sociodemographic characteristics of study participants, by study location. (N=1,127).

Overall, the study captured a spectrum of racial/ethnic identities, with 20% identifying Hispanic (and not multiracial), 42% as non-Hispanic White alone, 21% as Non-Hispanic Black alone, and 12% as other identities or multiracial. Most participants identified as male (63% vs 36% female), and 76% identified as straight while 22% identified as gay, lesbian, bisexual, or other. Forty-one percent of participants had attended at least some college or technical school. Over three-quarters of participants (78%) reported household income of less than $20,000, 37% reported public benefits or disability as their primary source of income (and 20% marked “other”), and 60% reported that they were currently experiencing homelessness or housing insecurity. Only 4% of participants reported possession of private insurance; most (64%) had only public or other insurance (Medicaid/Medicare/Veteran’s) and 24% had no insurance. Lack of insurance was highest in Atlanta (65%).

Thirty-one percent of respondents reported having exchanged sex for goods, a place to stay, money, or drugs/alcohol at some point during their lives, with the highest rate among Atlanta participants (45%). Most participants reported intravenous injection of drugs in the past 6 months (68%), with the highest rates among Las Vegas participants (68% vs 57% in Los Angeles and 67% in Atlanta). Twenty percent of participants had been incarcerated for five or more years in their lifetime, with 31% of Los Angeles participants reporting 5+ years compared to 12% in Las Vegas and 15% in Atlanta. The high proportion of longer incarcerations in Los Angeles reflects the older age of participants.

As shown in , all measured sociodemographic variables demonstrated statistically significant differences between cities, suggesting that these differences reflect differences in the populations of PWUD utilizing community partner services between these cities.

Vaccine confidence and vaccination receipt

Overall, participants who answered all eight scale items (missing = 129) averaged 12.4 (standard deviation 5.8) on the EVCI. Of these, 10% (n = 96) had high confidence in vaccines recommended for adults (EVCI 21–24), 35% (n = 348) had medium confidence (EVCI 13–20), and 56% (n = 554) had low confidence (EVCI 0–12). Confidence levels differed between sites (chi-square p-value = .005), with 9% of Las Vegas participants reporting high confidence, 39% medium confidence, and 52% low, compared to 13% high, 33% medium, and 54% low in Los Angeles and 6% high, 30% medium, and 64% low in Atlanta.

Self-reported vaccination receipt is estimated for each of the VC groups in , computed from the multiply imputed data. HepA, flu, MMR, and Tdap vaccination proportions were higher among medium and high confidence participants than among those expressing low confidence, though we did not detect differences in rates between medium and high confidence participants. The difference in vaccination receipt was most pronounced for Flu and Tdap vaccines, with 58.4% flu receipt for high confidence compared to 42.4% for low and 73.0% versus 57.0% for Tdap. We did not find sufficient evidence of differences in HBV receipt between VC levels.

Table 2. Vaccine receipt percentages by vaccine confidence class.

reports the bivariate and adjusted odds ratios of vaccine receipt for each EVCI point increase. With the exception of HepB, higher VC increased the odds of the vaccine receipt in both the bivariate estimate and the multivariable model adjusting for sociodemographic factors. The strongest effects were observed for flu vaccine receipt with a bivariate and adjusted odds ratio of 1.05 (corresponding to an EVCI of 24 associated with 3.1 times the odds of flu vaccination as an EVCI score of 0) and for Tdap, with an unadjusted and adjusted odds ratio of 1.04 (a factor of 2.7 difference between EVCI 0 and 24).

Table 3. Odds ratio for increase in likelihood of self-reported vaccine receipt per point of EVCI index.

Factors associated with vaccine confidence

Associations between VC and sociodemographic and other risk factors were explored by comparing bivariate group mean EVCI scores. Bivariate group means and 95% confidence intervals are shown in . Las Vegas participants had higher mean EVCI scores (12.4 95% CI: [11.9, 13.0]) than Atlanta participants (11.1 [10.3,11.8]). Respondents who were experiencing homelessness or housing insecurity had lower mean EVCI scores than those who were not (11.4 [10.9, 11.8] vs 12.6 [12.0, 13.2]). Additionally, recent use of intravenously injected drugs was associated with higher VC (mean EVCI 12.0 [11.6, 12.4], compared to 10.9 [10.1, 11.7] for those who had not recently intravenously injected drugs).

Table 4. Estimated mean EVCI index scores across sites and sociodemographic factors.

We also estimated factors associated with VC using a full multivariable linear regression (). After adjustment for sociodemographic and other risk factors location was still associated with differences in EVCI, with Las Vegas residence experiencing a difference in mean EVCI of 1.9 (95% CI: [0.7, 3.1]) compared to similar participants in Atlanta. As in the bivariate analysis, experiencing homelessness or housing insecurity was associated with reduced VC (−1.1 [0.3, −2.0] points). Public benefits or disability as a primary income source was associated with increased VC compared to participants whose primary income source was employment (1.4 [0.4, 2.4] point increase). Additionally, a lack of health insurance coverage was associated with a 1.5 [0.5, 2.6] point increase in EVCI compared to those with public forms of health insurance.

Table 5. Estimated increase in ECVI from full multivariable linear regression model.

Discussion

With data collected among PWUD just before the onset of the COVID-19 pandemic, we identified wide variation but low overall VC in this priority population. Similar to other studies that found comparatively lower VC in parents and caregivers of children,Citation41,Citation42 in sexual orientation and gender minority communities,Citation14 and other populations with considerable health disparities (i.e., immigrants, Black/African Americans),Citation68 it is somewhat expected that PWUD also held similar perspectives given the extent to which trust is encapsulated in VC.Citation41,Citation69 In general, like other segments of the community who have been marginalized and stigmatized, PWUD are distrustful of institutions, government, and authority that are distally responsible for creating the conditions and experiences of this community.Citation70,Citation71 As VC is shaped in relation to perceptions of such entities and factors, a decline in VC is inevitable without adequate response to ensure VC reinforcement and to counteract any further reduction in the realization of immunization goals.Citation72 Thus, the findings reflect the fundamental, yet enduring challenge that low VC presents as a barrier to adult immunization.

The findings offer insight on the role of VC in the achievement of greater immunization coverage. In particular, the findings reflect that receipt of Hep A, influenza, MMR, and Tdap (dTAP) vaccines were positively correlated with higher internalized VC. As these vaccines have long been available and feature prominently in childhood and adult vaccine recommendations, PWUD may hold greater VC for this set of vaccines due to lifetime exposure and experience with the products, thereby enabling the formation of a positive VC impression. As there have been repeated outbreaks of hepatitis A in communities with PWUD, provider recommendations, combined with immunization outreach, education, and vaccination events, may have shaped factors such as safety perception, trust in the recommendation, and perceived disease risk wrapped into the EVCI measure.Citation73–75 In contrast, the extent of VC held among PWUD did not correspond with reported HepB immunization. Given the extent of liver disease among PWUD, especially among older, racially/ethnically diverse men, the observed effect may correspond with ambivalent VC views associated with HepB vaccination to prevent chronic disease associated with infection.Citation76,Citation77 Thus, these findings offer insight on the sensitivity of our EVCI measures to detect vaccine-specific nuances affecting VC, uniquely shaped by lifetime experiences. Additionally, while VC was positively associated with vaccine receipt, the effect of vaccine confidence on receipt was not as stark as found among parents’ VC and childhood vaccination.Citation41 This suggests that among PWUD other factors are potentially acting to increase uptake among those with lower VC or depress uptake among those with higher VC, relative to childhood vaccination. Because uptake and VC varied across cities, it is possible that variations in experiences within public and government outreach programs may have a larger influence on uptake, moderating the role of VC. Among people experiencing homelessness, availability through routine access points as part of regular care is emphasized as a key facilitator of vaccine uptake, and regular engagement with health-care providers is associated with greater trust in the healthcare system.Citation78 Because we did not directly measure these experiences within our socioecological framing, this is an important dimension to incorporate into future research. However, VC remains important given its association with trust in health-care systems.Citation66

This study lays the foundation to develop a public health response consistent with this inquiry’s socioecological theoretical underpinnings. Our findings suggest that at the institutional level, nongovernmental, clinical-community interventions to improve VC should improve immunization uptake for recommended adult vaccines that will result in greater coverage rates among communities of PWUD.Citation70 At the community level, public health outreach and educational opportunities, particularly those led by trusted peers and directly conducted in community settings where PWUD congregate or where mobile social services are available such as needle exchange, would likely be highly effective in promoting vaccine confidence and uptake in the community. With peer leadership at the forefront of these efforts, combined with service provision, there is great potential to leverage community trust to promote understanding of disease risk and vulnerability, as well as offer direct access to lifesaving vaccines via onsite immunization clinics.Citation79 Such interventional strategies that combine elements of bringing positive vaccination messages forward from nongovernmental entities, involving trusted peers to deliver messages and provide vaccine access where the population is served have proven highly effective with other populations that have demonstrated lower confidence, vaccine hesitancy, or disapproval.Citation80–82

Our findings underscore that VC varies by geographic location. We identified Las Vegas, Nevada as having the highest VC of the three cities we surveyed (followed by Los Angeles, California and Atlanta, Georgia). These results are consistent with other studies that have identified geographic variance in VC, which in turn, may contribute to cluster outbreaks of VPDs among those who do not obtain vaccinations.Citation83–86 As VC is informed by lifetime navigation of social and structural barrier to health, our results likely reflect sociostructural and cultural differences that exist in each of these cities and socioecological dynamics may help explain the challenges and leverage points within the dynamic environment. Thus, the experience of a PWUD in a midsize urban city (i.e., Las Vegas) with government policies that favor direct access to 24/7 public health programs, resources (via storefront, vending machine, and mobile outreach), providers/outreach staff, may more favorably shape VC compared to larger, more impersonal, difficult to navigate, highly complex urban regions and health-care environments (i.e., LA, Atlanta). These nested levels of influence have often been linked in other vaccine confidence literature, pointing to the cascade of influences and experiences shaping community attitudes and vaccination behavior.Citation62,Citation87

We sought to identify mutable factors associated with VC given city differences and substantial place-based considerations. Against this backdrop, we examined sociodemographic factors that are associated with positive VC. Previous VC-related studies with diverse populations have suggested that social determinants of health such as educational attainment,Citation88 insurance status,Citation81,Citation89,Citation90 healthcare access,Citation91 employment status,Citation92 and income bracketCitation93 may be associated with the favorable VC and consequent immunization behavior in the larger population. Our findings are consistent with previous global studies that have identified populations who are housing- and/or food-insecure as less likely to have positive vaccination attitudes and to obtain immunization.Citation94–96 The stress associated with living under these circumstances, compounded by high unemployment challenges, not only impacts immunization considerations (i.e., obtaining seasonal flu vaccine), but other routine health-care decisions (e.g., physician visit, eye exam, routine labs).Citation96 Those who experience housing instability typically lack a home for usual (medical) care as well. As VC measurement incorporates consideration of provider recommendations and trust placed in the vaccine guidance, it is likely that persons experiencing these adversities are unable to develop VC compared to those with more stability in housing and employment.

Among the sociodemographic factors associated with higher VC, and associated with higher level socioecological effects (i.e., government/institutional policies and programs), PWUD with public benefits or disability pay and those without insurance exhibited greater VC. These individuals likely have their VC shaped by safety-netCitation97 health-care experiences, including emergency care (i.e., naloxone receipt), hospital emergency department (ED) encounters, and at public health facilities, syringe service programs, and/or visits made to free clinics. As most frontline safety-net entities include an inventory of receipt of routine vaccinations (e.g., flu, HBV, HPV),Citation98 and no-cost protection determination via titers and/or subsequent vaccine administration, at no cost to the individual given public assistance/disability status, more favorable preventive health-care views including VC may be cultivated during these experiences. With health-care staff who exhibit cultural competence, and remain sensitive to the needs of PWUD, safety-net sites have become a primary yet underutilized resource in further development of VC and approaches to surmount VPD inequities for vulnerable populations.Citation99–101 Additionally, such safety-net sites may provide opportunities for positive health-care interactions, fostering positive relationships and improved trust. Such positive relationships can improve healthcare engagement and VC.Citation66 For PWUD without insurance, this higher VC may suggest a pathway to improved vaccination rates, as we previously found that PWUD without insurance had lower rates of vaccine coverage than those with public or private insurance.Citation46 Further research is needed to determine the factors which influence higher VC among PWUD with different insurance coverage and to determine how community care organizations can leverage this confidence into vaccination opportunities.

Limitations

Due to confidentiality and logistical requirements, we were not able to confirm vaccine receipt or timing with state vaccination registries. Our community partners were concerned about the maintenance of client trust and confidentiality and therefore requested that we deidentify all data and not link any immunization or client data directly to these aggregated datasets. The self-reported nature of our vaccine receipt outcome means that these measures may be subject to recall or desirability bias, though these risks are somewhat mitigated by our screening process ensuring the clear-mindedness of participants and the private, anonymous, on-location and self-administration of surveys. Previous studies have found high sensitivity and specificity for self-reported influenza vaccination,Citation102 but lower specificity for vaccines such as HepB.Citation103,Citation104 Given the complexities around adult pneumococcal vaccination (i.e., new products, changing guidelines, PWUD access to providers who routinely recommend the immunization), and the additional participant burden it would place on survey participants, we did not incorporate this immunization in our assessment. Our interpretation of trust pathways leading to VC and the relationship between VC and vaccine receipt is limited by the logistics of the survey and anonymity requirements, as we do not have information about where and how recently vaccines were received, sequence and booster completion, or more detailed information about participants’ relationships with health-care providers and health information. Future studies should explore these relationships in more depth.

Future directions and conclusions

With a socioecological framework accounting for any array of nested levels of influences operating at the institutional/policy-, community-, and individual-level factors to shape PWUD VC, this study identifies barriers and facilitators to improvement of VC in this population and intervention points to surmount challenges for VC, vaccine uptake, and ultimately the achievement of broader adult immunization coverage. We identified several important subgroups with reduced vaccine confidence among participating PWUD, including those experiencing housing insecurity, those with employment as a primary income source, and those with public health insurance. Further research is needed to both determine the underlying causes of these associations and their generalizability to other PWUD across the US, as well as the potential moderating role of government and community health program engagement. Qualitative and mixed methods studies would be especially suited to this investigation. Given the overall prevalence of low vaccine confidence among PWUD, interventions to improve vaccine coverage should assess vaccine confidence and its potential impact on intervention effectiveness as part of their evaluation process. Interventions to improve vaccine confidence should be included in efforts to improve vaccine coverage among PWUD. Because of existing reach to PWUD, service organizations hosting syringe exchange programs could be important partners in efforts to improve vaccine confidence and vaccination rates among this vulnerable group.

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Acknowledgement

We are deeply appreciative to our community partners, including but not limited to TracB, Atlanta Harm Reduction Coalition, The Elizabeth Foundation, Bienestar Human Services, and Asian American Drug Assistance Program, and the clients of those agencies who volunteered their time to complete our survey. This endeavor was a collaborative effort that included a number of staff, including subject matter experts who assisted with instrument development and data collection procedures. We are also grateful for the guidance and insight of all members of HBOU project team. HBOU Project Team: Project Staff/Data Collection Team: Las Vegas: Mehret Girmay, John Olawepo, Sfurti Maheshwari; Los Angeles: Katy Berteau, Elizabeth Wu, Evan Kreuger, Ashleigh Herrera, Tasha Perdue, Mohammed Ahmed, Alberto Gonzales, Do Kin Luong, Cassandra DeWitt, Alicia Morales Perez, Francisco Rodriguez, Jade Dalton, Arthur Sun, Sarah Fiskin, Antonio Shallowhorn, Bernice Lopez, Alexandra (Alex) Michel; Atlanta: Stephanie Richardson, Allen Welty-Green, Priscilla Smith, Tracy Thompson. Community Partners: Las Vegas: TracB (Chelsi Cheatom, Rick Reich); Los Angeles: Bienestar (Robert Contreras, Joanna Barreras, Hugo Aguilar, Esmeralda Limeta); Asian American Drug Abuse Program, Inc. (Terri Reynolds); Atlanta: Atlanta Harm Reduction Coalition (Mojgan Zare, Mona Bennett); Elizabeth Foundation (Tracy Thompson). Subject Matter Experts: Max Gahk, JD; Sarah Hunt; Brian Labus; Ayako Miyashita, JD; Matthew Archibald.

Disclosure statement

Dr. Holloway acknowledges support from the National Institute of Mental Health (P30 MH58107) and the California HIV/AIDS Research Program (RP15-LA-007). Dr. Frew received internal funding from UNLV to facilitate partner engagement. Dr. Spaulding reports recent grants through her institution from the Gilead Sciences, the Bill and Melinda Gates Foundation and the National Institutes of Health; for a research project she has received test kits from Bioltyical Laboratories. These grants are not associated with the scope of inquiry for this study. Dr. Frew reports a relationship with Merck & Co: employment. Dr. Frew was faculty at University of Nevada and did not have a relationship with Merck & Co at the time the study was conducted.

Supplementary material

Supplemental data for this article can be accessed on the publisher’s website at https://doi.org/10.1080/21645515.2022.2123201.

Additional information

Funding

The work was supported by the California HIV/AIDS Research Program [RP15-LA-007]; National Institute of Mental Health [P30 MH58107].

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